The most common tuning system in western music nowadays involves dividing an octave into 12 equally spaced "semitones", called "equal temperament". In this system, the octave interval (frequency ratio 2:1) is the only "perfect" one, and, for example, a "perfect fifth" (frequency ratio 3:2) doesn't exactly match a "fifth" (seven semitones). This is the solution people came up with, and we've been using it happily for a while now, but there are other equally valid ways to split the octave. "Slendro", for example, divides it in a similar way with equally spaced intervals, but they do it with 5 instead of 12, which makes it sound very weird to our western ears.